1. Field of the Invention
This invention relates to a method and apparatus for detecting the presence of one or more harmful gases and, more specifically, to an improved gas detection apparatus which can accurately distinguish between various undesirable gases and generate a highly accurate electrical output signal indicative thereof which is compensated for sensor variations in conditions such as temperature, relative humidity, aging or drift, and the like.
2. Description of the Prior Art
Industry is extremely conscious of the ever-expanding need for more reliable, precise and faster responding gas detection instruments, primarily due to the impedance provided by the increasingly stringent EPA (environmental Protection Agency) and OHSA (Occupational Health and Safety Act) regulations both of which have estabished standards specifying that the air we breathe must be relatively free of dangerous contaminants and that the environmental atmospheres to which workers are exposed must be free of toxic, noxious, combustible and other hazardous substances which might endanger the worker's health or safety.
The growing public awareness of deaths and/or property damage which may have been prevented with proper detection or measurement equipment has led to increasing activity in this area, particularly in recent years.
The gas measuring and detection instruments of the prior art have proved, as a whole, grossly inadequate to meet the needs of today's industry and the more stringent demands imposed by governmental regulations and public outrage. The need for low cost instrumentation which is portable, accurate, selective, reliable, and easy to operate over relatively long periods of time has gone virtually unfulfilled.
In addition, rather than only intermittent testing, a detector is needed which operates continuously. This is important particularly in industrial applications so that a worker can carry the detector with him through a plant or wear it on his person to sense the presence of a potentially combustible, toxic, noxious or otherwise undesirable or dangerous gas and to provide an alarm indicative thereof. While permanently mounted fixed monitors may be placed in high-risk areas, the high-risk areas must be first identified with the use of continuously operating portable instruments. It is not practical to go through a plant initially with an instrument which must be repeatedly triggered to evaluate the atmosphere, and pockets of undesirable gases may be missed easily. Thus, continuous operation is an important industrial prerequesite for the gas detection instruments of today.
Portability and continuous operation becomes particularly important in applications such as waste treatment where the workers must operate in confined spaces such as sewers and manholes; fire departments which are required to conduct surveys in all types of confined and potentially dangerous areas; shipping where compartments, holds and confined spaces must be inspected; utilities where manholes and vaults are used; construction areas where workers operate in tunnels and other underground or confined areas; chemical operations which involve tanks, vats and the like; refineries involving process areas, holding tanks, treatment facilities and the like; mining where all types of combustible, toxic, or other hazardous gases must be detected in underground operations including remote pockets or low-lying areas of mines and the like; and inspection operations of any other type of enclosed areas, gas fields, etc.
Therefore, the need for a continuously operable, highly reliable portable gas detector usable in all such applications has existed for some time and continues unfilled to the present day.
Furthermore, modern gas detection instruments must be of a relatively low cost so that they can find wide usage in the various industries and not add greatly to the cost of the overall product or service provided. Ease of operation, operating time and life, and ease of maintenance are also important from these standpoints.
Modern gas detection instruments must additionally be highly accurate, extremely reliable, fast-acting, and, in most instances, must be selective, i.e., able to distinguish between one or more type of undesirable gas which may be present. These demands are of utmost importance due to the ever-increasing public demand for increased worker safety and clean air standards and additional incentive is provided due to the substantial financial penalties which may be assessed under the standards established by federal, state, and local governments for non-compliance therewith.
Most of the gas detector devices of the prior art suffer from one or more of the following drawbacks. The chemical types of gas detectors, including the calorimetric systems, are relatively slow-acting and are non-continuous. A sample of gas must be introduced through a powdery agents into a tube and a visible color change or stain occurs. The length of the stain must be physically measured to determine the relative concentration or contamination level. Thus, systems relying on chemical reactions are slow, non-continuous and costly. Furthermore, the tubes, once stained, are not re-usable.
Optical systems which rely on the infrared absorption spectra of a gas are not easily portable and are too expensive to install at even fixed locations in a plant. In the absence of portability, continuous sampling and prompt indication of toxicity levels, optical systems are not feasible for the widespread usage required under today's conditions.
Another type of system utilized in the prior art to detect the presence of toxic gases is gas chromotography in which a sample of gas is injected into an absorption column and the reaction is timed and absorbed. Thus, a slow response and non-continuous sampling are obvious drawbacks of gas chromotography systems. Such systems are non-portable, highly costly and not usable in most of today's industrial applications under discussion.
Still another prior art system is electrochemical gas detection wherein an oxidation-reduction reaction takes place in the fuel cell with the gas being sampled serving as one electrolyte. The fuel cell generates a current proportional to the electrolyte concentration and this type of system requires a pump and filter, reference electrodes, oxidation and reduction electrodes, feedback voltage circuitry and read-out circuitry as well as the secondary electrolyte, etc. This type of system, however, cannot selectively identify carbon monoxide, is quite complex and not generally satisfactory for all types of applications. Furthermore, such a system is normally unable to select between different types of gases, is relatively costly, and its readings are not without error induced by various conditions such as changing temperatures, humidity, aging and the like.
It is not enough that modern systems be highly accurate, reliable and continuously operable, but portable units must also be able to operate without interruption for recharging for at least an eight hour period and the system must operate continuously to be able to detect small pockets or intermittent emissions of undesirable gases which periodic sampling would be unable to detect with any degree of reliability.
One of the most recent and most popular types of gas sensors for detecting the presence of various gases is the semiconductor catalytic gas-sensing element such as the Taguchi gas sensor, made by the Figaro Engineering Company, Inc. of Osaka, Japan, and which is disclosed in U.S. Pat. No. 3,676,820, which is incorporated by reference herein. Similar types of semiconductor catalytic sensors are now available and could be used as well. Such sensors are typically composed of a bulk n-type metal oxide such as tin oxide, zinc oxide, etc. and the sensor decreases its electrical resistance when it absorbs the oxidizing or combustible gases such as hydrogen, carbon monoxide, methane, propane, alcohol, volatile oil and a settling as well as carbon-dust containing air or smoke. This decrease is usually large enough to be measured without amplification.
For these reasons, the semiconductor catalytic-type gas sensor has found wide usage as an alarm device and such applications as fire alarms, alcoholic breath analyzers, gas leak detection, ventillator control, etc. As pointed out in U.S. Pat. No. 3,906,473, one difficulty with gas detectors using such elements is that, with the sensor operating in its high temperature range, its response to carbon monoxide and methane is substantially lower than for other more complex hydrocarbon molecule gases. Furthermore, the sensitivity of the equalibrium method is less than is desired for some purposes and, in fact, is not sufficiently accurate or reliable for many of the applications required in industry.
Furthermore, it has been found that such semi conductor catalytic gas-sensing elements which utilize the principle whereby the electrical resistance or some similar property changes with concentration of a particular gas to which the sensing element is susceptible have proven to be unreliable and give erroneous information since the same electrical property such as resistance is found to change significantly with such external conditions as temperature, relative humidity, aging or drift, and the like, thus rendering gas detection devices employing these gas-sensing elements relatively unreliable and inaccurate under many sets of circumstances frequently encountered in industry and in fact dangerous if reliance is placed on the accuracy thereof without knowledge of the conditions which can render the readings erroneous.
It is also desirable to be able to detect either a first or a second undesirable gas and some level of concentration of the sum of these gases with the same sensing elements, wherever possible, but this has heretofore been unachievable with any degree of accuracy and reliability, particularly in the portable, low-cost, continuously operating detection systems required today.
Furthermore, the prior art exhibits a need to combine gas detection instruments for sensing combustible, toxic or other noxious gases with a compact system for sensing oxygen deficiency but the standard prior art units utilizing standard detection cells such as the micro fuel cell manufactured by the Teledyne Instrument Company usually become inoperative below some predetermined lower temperature operating limit such as 32 degrees Fahrenheit and it is desirable to be able to utilize today's gas-sensing instruments at significantly lower temperatures.
The present invention eliminates nearly all of the deficiencies of the prior art gas detection systems and provides a self-compensating apparatus for detecting and identifying one or more undesirable gases and produce a highly accurate and reliable electrical output signal indicative thereof, the output signal being compensated for errors due to variations in conditions such as temperature, relative humidity, aging or drift, and the like.